This laboratory was the first to transmit non-A, non-B hepatitis (subsequently proved to be hepatitis C) and human immunodeficiency virus (HIV) to the chimpanzee and hence to establish an animal model for these infections. Current studies in this model include the following: The Early Events of HIV Infection: In this study a chimpanzee was infected with HIV and serial apheresis units were obtained during the window period between exposure and the first detection of anti-HIV antibody. It was shown that no markers of HIV infection were detectable until the fifth week post-exposure when virus was detectable by HIV RNA, HIV DNA, and viral culture. Anti-HIV and p24 antigen, as used in blood donor screening, were not detectable until 8 weeks post-exposure. Subsequently, the 3-, 4-, and 5-week samples were sequentially inoculated into a second chimp. The 3- and 4-week samples were shown to be non-infectious, while the 5-week sample was infectious. Hence, infec-tivity did not precede the detection of HIV RNA and DNA. This suggests that if such assays were introduced into blood screening, they might totally abrogate the infectious window and prevent blood transmission of HIV. Viral Inactivation: In collaboration with Cerus Corp., the chimp model was used to establish the efficacy of psoralen/UV-inactivated platelets. This is the first viral inacti- vation procedure that maintains the integrity of the cellular components of blood. Three chimpanzees have each been exposed to infectious doses of hepatitis C virus (HCV) and hepatitis B virus (HBV) that have been psoralen-UV treated. The study is still in progress, but after 9 months of follow-up, no animal has been infected with either HBV or HCV. These animal studies confirm in-vitro efficacy data and set the stage for safety and effi-cacy trials in humans. This method should have broad application for platelet transfusion therapy and, ultimately, for red cell transfusion as well.